Its a cable connect like in the pic you posted. the upside to the flex cable is it would allow the boards to be closer than if using pogo pins. thus allowing a thinner profile. Also it might allow use of spring pins on the pads for pins 24 to 33, meaning more pins for other uses.
Now I just need to find my teensy 4s I have. Seem to have misplaced the box my teensy 4 project was in.